Modern computers are fast enough that opening applications are mere seconds. I just did a test on my box.
Maybe Safari, TextEdit, etc. Try Photoshop, InDesign, even Word, or anything else that loads a bunch of plugins or font's when you start it.
Personally, I don't make the mistake of leaving 10 apps open. I see it happen sometimes to others, but I think that only happens to switchers and novices. Normal mac users and pros quit their apps properly just like normal windows users close the last window instead of minimizing (or leaving it behind the other windows that they insist on maximizing). Anyone can leave something running in the dock or start bar by accident.